[27.02] Observation of current sheet pinch in a solar flare

High-cadence and high-resolution time sequences of far
H-alpha off-band images provide a unique tool to study the
evolution of the fine structure of flare kernels. The fine
structure contains important information on flare topology
and the triggering mechanism. In this paper, we concentrate
on the rapid changes of the relative positions of two
conjugate flare footpoints. In order to carry out this study
with the highest physical precision, we use rc = \Sigma
rj Ij / \Sigma Ij (Ij is the H-alpha brightness at
rj) to compute the centroid of an H-alpha bright kernel
region caused by solar flares. Using this, we probe the fine
temporal structures connected to the distance between the
centroids of two conjugate kernels of an M2.3 flare. The
flare, which occurred on 2002 September 9 in active region
NOAA 0105, was observed at Big Bear Solar Observatory (BBSO)
at the far off-band center wavelength of H-alpha - 1.3 Å,
with a cadence of ~ 40 ms. The flare was also observed
by the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI). The time profile of the separation
distance shows an excellent anti-correlation to that of the
hard X-ray (HXR) emissions in 25 - 50 keV, which exhibit a
number of separate spikes (The linear Pearson correlation
coefficient is found to be ~ -0.83). The separation
between the two centroids decreases at the rising periods of
four HXR spikes, then it increases after the peak time of
the flare to show the expected separation motion. The most
obvious decreasing, which occurred during the first HXR
peak, was confirmed by corresponding images. This implies
that during the impulsive phases, the energy transported
from the corona is deposited increasingly inwardly between
the two kernels. This new, and perhaps surprising tendency
for the energy deposition can be explained as being caused
by current sheet pinch motions, which, at the same time,
enhance the magnetic energy reconnection rate to produce the
observed HXR spikes.